The present invention relates to grinding machines and will be disclosed in connection with a control apparatus which varies the infeed rate of a tangential grinding machine to optimize grinding operations whenever the grinding wheel or workpiece sizes are variable.
Tangential grinders are known in the art and are characterized in that the workpiece feed vector is noncoincident with the rotary axis of the abrasive wheel. Generally, the relative infeed motion between the grinding wheel and workpiece in a tangential grinder is tangent to the periphery of the grinding wheel axis. The relative motion may be accomplished by the workpiece feeding tangent to the periphery of the grinding wheel or vice versa. Alternatively, the motion between these two elements may be skewed. The most prominent contemporary example of a tangential grinder is a surface grinder.
Like all grinders, tangential grinders are plagued with the problem of wheel breakdown, a problem which is virtually inherent in the grinding process. This breakdown is a function of the force generated between the grinding wheel and workpiece in machining operation. It is commonly expressed in terms of stock removal rate per inch and G-ratio. As a consequence of wheel breakdown, the abrasive wheel dimensions vary from the their original size to some smaller "stub out" dimension. Depending upon the original size of the abrasive wheel, the dimensional change may be quite substantial. For example, a wheel with an original 36 inch diameter might be used upon a machine until a "stub out" diameter of 26 inches is reached, utilizing five inches of wheel radius.
Prior art tangential grinding machines move the workpiece relative to the abrasive wheel at a fixed velocity during the period these two elements are in grinding engagement. This fixed velocity feed has resulted in several problems in tangential grinding. As the grinding wheel is reduced in size, workpiece contact with the wheel is delayed as the workpiece (or grinding wheel, as the case may be) must be moved through a greater distance before contact with the grinder wheel (or workpiece) commences. Thus, a greater portion of the feed stroke is non-productive (grinding air). If the rapid advance portion of the stroke is fixed, significant increases in non-productive cycle time result without offsetting advantages. Additionally, and perhaps more significantly, prior art machine control concepts have required exceedingly high stock removal rates in the diminished cutting path length whenever a large amount of stock is removed with a worn wheel. This has resulted because the prior art machines have fixed feed velocities through variable cutting path lengths, the cutting path being reduced in length in proportion to the grinding wheel wear. Further, stock material often fluctuates in its dimensions. Hot forged cylindrical workpieces, for example, often exhibit wide variations in their diameters. Thus, the cutting path often fluctuates even with a constant wheel size and is not predictable.
Prior art machines have been relegated to compromising between one of two trade off situations. If the grinding feed velocity is set for a new wheel size, exceedingly high stock removal rates will be experienced after the wheel undergoes wear. While the initially suitable, relatively rapid, feed rate reduces cycle time, continued use of this rate may result in multiple problems after the wheel experiences wear. Such problems may include, inter alia, accelerated wheel breakdown, depreciated workpiece finish and even workpiece sizing problems. On the other hand, if the machine rate is set for the worn wheel size, reduced cycle time will be experienced with a new wheel, and a slower cycle time will result. Quality control considerations frequently dictate that the worse case situation be accomodated; and this latter, slower cycle time is frequently adopted.
Applicant has alleviated the above mentioned problems with a novel and unique control scheme that contains the advantages of each of the formerly available trade off situations while eliminating the disadvantages associated with each. Consequently, many of the compromises inherent in prior art machines are removed and a new and improved grinder control results.